1. Field of the Invention
This invention relates generally to electromagnetic sonar transmitters. More particularly, the invention relates to an electromagnetic sonar transmitter apparatus and method which eliminates the prior art requirement for bias magnetization by utilizing driving signals having frequencies offset from the frequency of the radiated acoustic signal.
2. Description of the Prior Art
Sonar systems detect and characterize objects in a liquid medium by first impressing an acoustic signal into the medium and subsequently analyzing the returning echo. In order to provide the necessary acoustic signal, various transmitters have been developed. Typically, these transmitters include a signal source operating a submerged acoustic projector comprising some type of signal transducer.
One type of acoustic projector transducer well-known in the prior art is the piezoelectric transducer. These transducers utilize piezoelectric elements which deform upon the application of a voltage to produce an acoustic signal. Piezoelectric transducers, however, have been found to have significant drawbacks. For example, they become massive and complex at low frequencies. Further, piezoelectric transducers are susceptible to performance variations depending on depth.
Another type of projector transducer, which has been found, for example, to be more suitable for operation at lower frequencies than the piezoelectric transducer is the electromagnetic transducer. Electromagnetic transducers are typically constructed having at least one movable electromagnet affixed to a radiating surface. A driving signal applied to the electromagnet produces a magnetic attraction force urging displacement of the radiating surface. This magnetic attraction force has generally been derived by prebiasing the electromagnets and driving them at the desired frequency with a controlled voltage or current source. Bias magnetization, which may be provided by DC electromagnets or, equivalently, permanent bias magnets (or both), was believed necessary in order to make operation of the electromagnetic transducer linear.
The use of bias magnetization in electromagnetic transducers introduces a number of undesirable characteristics. For example, requirements for linearized operation with low harmonic distortion severely limit the amount of magnetic field fluctuation about the prebias field level. This is due to the fact that large excursions of the electromagnet may cause its respective pole faces to pull and stick together. Furthermore, the use of a permanent magnet for prebias may enhance sensitivity to operating depth because increases in external hydrostatic pressure causes structural compliance of the projector. This, in turn, decreases the separation between the pole faces of the electromagnets which can result in sticking, as described above.